A magnetic disk unit, such as a hard disk, is provided with a magnetic disk which has a plurality of tracks concentrically arranged thereon, and each track has servo data written therein. The servo data contains address data and burst signals to be used for position control of the magnetic head.
One of the known methods for writing servo data is so-called self servo write (SSW), which writes servo data into the magnetic disk by controlling the magnetic head and actuator, which are accommodated in the enclosure, after the magnetic disk unit has been assembled.
The problem involved in recording servo data in a magnetic disk is that air flows produced by the rotating magnetic disk shake the support system of the magnetic head, thereby forming distorted tracks on the magnetic disk. The distorted tracks are a main cause that obstructs the positioning of the magnetic head.
Japanese Laid-open Patent No. 2006-40423 (Patent Document 1) discloses a technique to carry out self servo write while the enclosure is filled with helium which has been introduced from a gas inlet (hole) passing through the enclosure of the magnetic disk unit. The magnetic disk in the enclosure filled with helium experiences relatively little shaking and hence permits nearly round tracks to be formed because helium has a lower density than air.
Unfortunately, the technique disclosed in Patent Document 1 requires that introduction of helium should be carried out in an environment (such as clean room) with a high degree of air cleanliness in order to prevent particles from entering the enclosure through the gas inlet hole. This imposes restrictions on the manufacturing process.